Corona discharges are used widely in industry to promote adhesion between various materials. In manufacturing polymer coated paper products, there is a large body of literature describing various applications of coronas to make polymeric coatings adhere to paper substrate materials. Almost all of these coronas are produced by applying a high voltage (approximately 5-10 kV), typical frequency (10 kHz) signal to electrodes in air at atmospheric pressure. See, for example, U.S. Pat. No. 3,411,908, 4,147,836, 4,220,471, 4,476,153, or 5,466,333. There are limitations to the usefulness of corona treatments, however. Coronas produce locally energetic discharges, known commonly as streamers, and these streamers may cause a non-uniform level of treatment. Furthermore, although coronas are generally effective at promoting adhesion of polyethylene to paper, they are relatively ineffective at promoting the adhesion between paper and other polymers such as PET, PEN, etc. Thus, there remains a need to improve adhesion between paper and other polymers.
An alternative method of pretreating paper for polymer coatings is with low pressure glow discharge treatments. Glow discharges are, by nature, very diffuse and homogeneous, producing a more uniform treatment. A major disadvantage in glow discharge treatments done at reduced pressures is the problem of maintaining a low pressure at the treatment station. This is an especially acute problem for paper, since its porous structure contains air and is continuously degassing during the treatment process. It is necessary to use either a batch process, in which the paper is loaded into a chamber and the air is removed, or an in-line process, which requires that the support pass through a differential pressure region. In the first case, the support must go through an additional offline step before the coatings are applied. This is unattractive from a product-flow perspective and requires additional capital. The second choice is difficult and expensive to implement because of the very tight tolerances needed to maintain the pressure differentials in the transport path. This requires expensive and complicated hardware and pumps. The closer to atmospheric pressure that these treatments can be done, the simpler and less costly the process becomes.
It is known that under the right conditions, stable diffuse glow discharges can be produced at atmospheric pressures. Work in this area has been limited and directed primarily at etching of photoresist and deposition of materials. However, there are references to treatments for adhesion (WO 94/28568). Many reports indicate that a reliable method of producing diffuse glow discharges at atmospheric pressures is to use helium as the discharge gas. Furthermore, Yoshikawa et al (U.S. Pat. No. 5,316,739) have described devices to be used to treat rubber at atmospheric pressure with helium and mixtures of helium with other gases. They use power supplies ranging in frequency from dc to 13.56 MHz, and show no advantage to be gained by using a particular frequency or small range of frequencies.
In U.S. Pat. No. 5,558,843 a method of treating a polymeric support comprising a first electrode having a first surface, the first electrode having a plurality of spaced apart holes adjoining the first surface, positioning a second electrode, having a second surface spaced apart from the first surface of the first electrode, pumping gas through the holes wherein the gas is greater than or equal to atmospheric pressure, the gas comprising helium and optionally oxygen and/or nitrogen, coupling power to the first electrode having a frequency of 10 kHz to 50 MHz, and positioning a web between the first surface of the first electrode and the second surface of the second electrode wherein the polymeric web is subjected to atmospheric glow discharge to improve the adhesive properties was disclosed.
The above method has been found to be very useful, but it is quite important in photographic systems to be able to run paper at extremely fast rates, such as 30 meters per minute or above and at comparatively low power densities, such as 5 joules per square centimeter or less.
In U.S. Pat. No. 5,714,308 was described a preferred combination of treatment gases and treatment frequencies that allow the use of the above device at higher than expected speeds and lower than expected power densities. Specifically, the gas comprises helium and optionally oxygen and/or nitrogen. Particularly effective and fast treatments are possibly when the power supply operates at a frequency of between 40 kHz and about 500 kHz.
One disadvantage of using the technique described in the aforementioned filings is the need for a treatment electrode fabricated with a plurality of closely-spaced holes. Making such an electrode requires precision machining, leading to considerable cost. Furthermore, it is matched to a particular drum electrode, and is not interchangeable with other diameter drums. There would be considerable advantages to being able to use standard corona discharge treatment devices, many of which are already in-place throughout photographic manufacturing operations, to achieve the same results described in these previous filings.
The present invention solves the above mentioned problems and provides a method for improving the adhesion between photographic paper and polymer coatings such as polyethylene.